Apparatus for cutting sections from well casings



P 11, 1962 G. B. CHRISTOPHER 3,053,182

APPARATUS FOR CUTTING SECTIONS FROM WELL CASINGS Filed April 4, 1960 2 Sheets-Sheet 1 Glenn B. Christopher ATTORNEY p 1962 G. B. CHRISTOPHER 3,053,182

APPARATUS FOR CUTTING SECTIONS FROM WELL CASINGS Filed April 4, 1960 2 Sheets-Sheet 2 Fig.7

INVENTOR Glenn 8. Christopher BY W ATTORNEY 3,053,182 APPARATUS FER (PUTTING SECTIONS FROM WELL CASINGS Glenn B. Christopher, Arlington, Tex, assignor to Jet Research Center, Inc., Arlington, Tex., a corporation of Texas Filed Apr. 4, 1960, Ser. No. 19,954 7 Claims. (l. 102-20) The present invention relates to apparatus for cutting sections from well casing in well bore holes. It is more specifically related to an improvement in an apparatus utilizing shaped explosive charges to cut a section of well casing into small pieces that may be removed through the well casing if desired.

This invention is an improvement in the invention set forth in the application of George C. Howard et al., Serial No. 372,905, filed August 7, 1953, entitled Apparatus for Cutting Holes in Well Casing, now US. Patent No. 2,935,020, issued May 3, 1960, and assigned in part to a parent corporation of the assignee of the present invention.

Quite often, in the course of drilling or completing an oil or gas Well, it is desirable to remove a section of the Well casing at a considerable depth in the well. The need to remove a section of Well casing most commonly occurs where it becomes necessary to offset an original Well bore by a sidetracking operation. Sidetracking of the original well bore may be required because of stuck drill pipe or bit, broken drill pipe or other down-hole equip ment, washouts or numerous other occurrences that require abandonment of at least a part of the existing well bore hole. Previous methods of removing sections from a well casing included mechanical milling or cutting out a section, or dissolving away a section by using strong mineral acids. These methods are time consuming, expensive, and often do not produce an opening in the casing suitable for a sidetracking operation.

Shaped explosive charges have found ready acceptance and wide application as a means of perforating well casings and the adjacent oil and gas producing earth formations. Annular shaped charges are frequently used to sever casing that is cemented in the well bore or drill pipe that has become stuck during the drilling of a well. Attempts to utilize shaped explosive charges to make intersecting cuts in metal plate or tubing to remove sections therefrom have encountered the problem of interference between the vertical and horizontal jets. When two linear shaped explosive charges are positioned in such a manner that when detonated simultaneously their jet streams intersect, the jet streams interfere at their point of intersection and the metal target may not be cut through at this point.

Thus, to successfully cut a length of well easing into small sections that can pass through the uncut casing, it is necessary that the horizontally and vertically acting shaped charges be arranged and detonated in a manner to provide a minimum of interference between their jet streams.

Therefore, it is an object of this invention to provide an improvement in an apparatus utilizing shaped explosive charges to cut a selected length of a Well casing or the like into small sections that can pass through the uncut Well casing.

Another object of this invention is to provide an improvement in an apparatus utilizing an alternating series of annular and frusto-spherical lined shaped explosive charges whereby the interference between the intersecting vertical and horizontal jet streams is minimized to insure complete severance of each casing section or the like from the adjacent sections.

A further object of this invention is to provide a novel Patented Sept. 11, 1962 detonating assembly in a shaped charge apparatus for making intersecting horizontal and vertical cuts in well casing or the like whereby the adjacent horizontal cutting charges are caused to 'detonate before the intermediate vertical cutting charge to produce a complete cut-through of the well casing or the like at the points of intersection of the respective jet streams.

Yet another object of this invention is to provide an improvement in a shaped charge apparatus for cutting sections from Well casing or the like comprising a unique coupling arrangement for associating a plurality of shaped explosive charges together. Such an arrangement permits a quick assembly of the various components into a shaped charge apparatus which can include as many individual shaped charges as desired.

The apparatus of the present invention includes axially aligned first, second and third sections of detonating explosive. The first and third sections may be generally annular in shape and have circumferential recesses in their peripheries. The second section may be substantially frusto-spherical in shape and has a multiplicity of longitudinal grooves therein. The grooves may take the form of longitudinally convex and circumferentially concave recesses. Metal liners may line the recesses in the first, second and third sections of detonating explosive. All of the sections of detonating explosive are spaced along their common axes so that the cuts in the casing produced by the jets of the second detonating explosive intersect the cuts in the casing produced by the jets of the first and third explosive sections. The respective explosive sections are adapted to be detonated within a short time interval with a delay in time provided between the detonation of the first and third sections, on the one hand, and the second section, on the other hand, to prevent interference between the jets provided by the several sections. The length of the explosive assembly may be extended by adding repetitive explosive sections.

The manner of construction and mode of operation of exemplary apparatus embodying the invention and other aims and objects of the invention will appear in the following detailed description.

Referring to the drawings:

FIG. 1 is a vertical, axial, sectional view of a shaped charge apparatus for cutting sections from well casing in accordance with the present invention, the apparatus being shown as suspended in a well bore;

FIG. 2 is an enlarged vertical sectional view of the upper or first annular shaped charge assembly;

FIG. 3 is an enlarged vertical sectional view of a por* tion of the apparatus of FIG. 1 showing the intermediate and lower, or second and third, shaped charge explosive sections coupled into a unitary second shaped charge assembly;

FIG. 4 is a plan view taken along line 44 of FIG. 3, looking in the direction of the arrows;

FIG. 5 is a plan view taken along line 5-5 of FIG. 3, looking in the direction of the arrows; 7

FIG. 6 is a vertical sectional view of a booster explosive ring used in the apparatus of FIG. 1; and

FIG. 7 is a plan view of the booster explosive ring of FIG. 6.

Referring now to the drawings, in which identical numerals are employed to designate identical parts, there is depicted in FIG. 1 a well casing .10, in which a housing 11 is suspended from the earths surface by the usual conductor cable (not shown). The housing is a closed, hollow, generally cylindrical, liquid-tight assembly including a wall member :12, a bull plug 13 closing the bottom opening of the wall member, and an annular member 14 closing the top opening. The wall member is attached to the closing members by cap screws 15 with neoprene O-rihgs 16 providing liquid-tight seals between the wall member and end closing members. Alignment spring wires 17 are carried by the end closing members to maintain the explosive charge apparatus axially centered in the well casing. The bull plug 13 and the annular top closing member 14 are preferably made from a frangible metal or metal alloy such as Zamac (a zinc base alloy), high silicon aluminum alloy or cast iron. Upon detonation of the explosive shaped charge assembly, these end closing members are broken up into small pieces which will not bridge or lodge in the well casing. The hull plug is provided with a large cavity -18, which presents a large surface area for the explosive forces to act upon to enhance the fragmentation of this member. The wall member 12 is a cylindrical tubular section of thin wall aluminum alloy or mild steel. For wells of intermediate depth where hydrostatic and formation fluid or gas pressures do not exceed 5,000 p.s.i.g., an aluminum alloy section with a wall thickness of about in. has proven satisfactory. Where down-hole pressures of 5,000 to 10,000 p.s.i.g. are expected to be encountered, mild steel of about W in. wall thickness is preferred.

The housing 1 1 is attached to a firing head assembly 19 by means of an externally and internally threaded collar 20 extending above and formed integrally with the annular top closing member 14. The annular top closing member and threaded collar are provided with an axial bore opening 21 therethrough which is counterbored and internally threaded for a portion of its length. The firing head body 22 provides an axial bore opening therethrough and is counterbored at its lower opening. The walls of the counterbore are threaded and receive the threaded collar 20 of the annular top closing member 14. A neoprene O-ring 23 provides a seal between the firing head body and the threaded collar of the annular top closing member. The upper portion of the firing head body is of reduced diameter and threadably engages a sleeve 24. A cable connector 25 is received in the sleeve 24 and provides an upwardly facing shoulder 26 which engages a downwardly facing shoulder 27 on the sleeve to retain the cable connector within the sleeve. The cable connector is provided with an axial bore opening therethrough and has a counterbore in the lower extremity. A blasting cap holder 28 is received in the cable connector counterbore and threadably engages the wall of the axial bore opening at the lower end thereof. An axial bore opening is provided in the blasting cap holder to receive the electric blasting cap 29. Insulated electric lead wires 30 from the blasting cap pass through the cable connector bore to the insulated core of the supporting cable (not shown) and to an electrical energy source on the surface. The upper extremity of the firing head body 22 is provided with two annular grooves which receive neoprene O-rings 3-1 in sealing engagement with the lower extremity of the cable connector 25. A booster blasting cap 32 is received in the upper end of the bore opening of the firing head body in detonating relationship to the electric blasting cap 29. Several wraps of electrical insulating tape 33 about the upper end of the booster blasting cap prevent it from sliding down the bore opening in the firing head body. A length of detonating fuse 34, such as Primacord, is attached to the lower end of the booster blasting cap. The detonating fuse extends through the firing head body bore opening, the hollow metal spindle 41, the cap locking ring 57 and the hollow locking pins 54, and terminates in the bull plug cavity 18.

Upperrnost in the housing is a first shaped charge assembly 35. Referring now to FIG. 2, the first shaped charge explosive section 36 has a generally annular recess 37 in the periphery thereof. A metal liner 38 lines the recess in the periphery of the explosive section. An explosive booster ring 39 is axially aligned in a groove in the central bore of the first detonating explosive section. An explosive booster plug 40 adjacent the explosive booster ring extends radially inward from the ring. A hollow metal spindle 41 projects through an axial opening provided in the first detonating explosive section. The locking nut 42 threadably engages the spindle, coupling the explosion confining disks 43, spacing disks 44, and first explosive section into a unitary assembly. A radially projecting shoulder 45 on the lower end of the spindle retains the axially aligned components of the first shaped charge assembly on the spindle. The spindle provides a transverse opening to retain the booster plug. The first detonating explosive section 36 is formed by adhesively joining two independently formed half-sections of a detonating explosive such as desensitized Cyclonite. The half-sections are prepared by pressing a suitable amount of granular explosive into a properly shaped mold containing a half-section of the metal liner so that the explosive adheres to the liner half-section. Each explosive half-section is provided with an axial opening therethrough to receive the spindle 41 and also provides a recessed surface adjacent to the axial opening to receive the explosive booster ring 39, as can be seen in FIG. 2. In FIGS. 6 and 7, the explosive booster ring is illustrated in greater detail. The booster ring is prepared by pressing a charge of shock sensitive explosive such as unwaxed or pure Cyclonite 46 in a suitable mold to form a ring shaped structure surrounded by a thin metal band 47.

The explosive booster plug 40 is formed by pressing unwaxed Cyclonite in a suitable mold. Covering the upper and lower plane surfaces of the first detonating explosive section are the explosion confining disks 43. These disks are made of metal, preferably lead, and serve to direct in a radial direction the force generated by detonation of the explosive. Immediately adjacent the explosion confining disks are the spacing disks 44, which are preferably of plywood, but may be made from wood, hardboard such as Masonite, or a rigid thermosetting plastic material such as Bakelite. Covering the bottom plane surface of the lower spacing disk is another explosion confining disk 43 which is placed in an explosive confining relationship with the upper plane surface of the hereinafter described second shaped charge assembly 48 when the various shaped charge assemblies are interconnected.

As illustrated in FIG. 1, immediately below and in axial alignment with the first shaped charge assembly 35 is a second shaped charge assembly 48. Referring now to FIGS. 3 and 4, the second shaped charge assembly has a second detonating explosive section 49, substantially frusto-spherical in shape with a number of longitudinally convex and circumferentially concave recesses 50 in the spherical surface. A metal liner 51 lines the walls of the recesses. Radially extending, longitudinal metal fins 52 are attached to the liner between the recesses 50. A number of axially aligned booster rings 39 are disposed centr-ally in the second detonating explosive section. A locking member 53 is centered and embedded within the top of the second detonating explosive section and flush with the top surface thereof. Explosion confining disks 43a are interposed between the second detonating explosive section and a third detonating explosive section 36a, which is generally annular in shape with a circumferential recess 37a in the periphery thereof. A metal liner 38a lines the recess. An axially aligned booster ring 39 is received in a groove in the central bore of the third detonating explosive section. A booster plug 40 adjacent the booster ring extends radially inwardly from the ring. Two alternate explosion confining disks 43 and an intermediate spacing disk 44 are positioned subjacent to the third detonating explosive section. A hollow metal locking pin 54 is received in axial openings in the second and third detonating explosive sections, the spacing disks, and the explosion confining disks. The locking pin is threadably engaged at its upper end by the locking member 53. A radially projecting shoulder 45 is provided on the lower end of the locking pin to retain the components of the second shaped charge assembly on the locking pin. A transverse opening is provided in the locking pin to receive the booster plug 40. The locking member 53 is provided with a shallow cylindrical recess 55 in its upper "'5 surface which receives the shoulder 45 of the spindle 41 to retain the first and second shaped charge assemblies in axial alignment.

The second detonating explosive section 49 may be made from any detonating explosive such as pentaerythritol tetranitrate (PETN), trinitrotoluene (TNT) or mixtures thereof such as Pentolite (50% PETN-50% TNT) by casting the explosive in the metal liner 51 enclosed in a suitable mold or from a compressed granular explosive such as Cyclonite by pressing the explosive into the metal liner held in :a suitable die. The metal liners 38, 51 and 38a for the detonating explosive sections are made from relatively thin metal such as copper, steel, brass or aluminum. Copper is preferred because it produces jets of increased penetrating ability. The fins 52 are attached to the metal liner of the second detonating explosive section by soldering or brazing. The third detonating explosive section 3611 is formed of the same explosive composition and in the same manner as hereinbefore described for the first explosive section 36, and is similarly lined. Explosion confining disks 43a are of the same material and shape as explosion confining disks 43 except that they provide a larger diameter axial opening to hold a booster ring 39 in detonating relationship between the second and third detonating explosive sections.

FIG. illustrates the lower half-section of a third detonating explosive section 36a with the associated liner 38a, booster ring 39, booster plug 40 and hollow locking pin 54.

As can be seen in FIG. 3, the hollow locking pin 54 and the locking member 53 are threadably engaged to hold all the components in a rigid unitary second shaped charge assembly 58. Referring to FIG. 1, any number of second shaped charge assemblies may be axially aligned below a first shaped charge assembly 35 in the housing 11 to provide an explosive apparatus of any convenient length for cutting well casing into sections' About 15 to feet of well casing is ordinarily removed to permit setting of a whipstock tool to sidetrack a well bore hole. Utilizing the present invention, an explosive apparatus of 15 to 20 feet in length can be easily assembled and will permit cutting of this length of easing into small sections in one operation. The provision of radially extending, longitudinal fins 52 on the liner for the second detonating explosive section gives a twofold advantage. As can be seen in FIG. 4, the plurality of fins are in substantial contact with the housing wall member 12, thus lending structural support to the wall member to resist inward deformation when exposed to high hydrostatic well pressure. This feature permits the use of a thinner wall member with consequent increase in the cutting energy of the jet reaching the Well casing. A second advantage provided by the fins is in facilitating the assembly of the explosive apparatus by centering the plurality of second shaped charge assemblies within the housing 11. Thus, in building up the complete explosive apparatus, each separate second shaped charge assembly is axially aligned with that assembly immediately above and below, and interlocked thereto by virtue of the shoulder 45 of the locking pin 54 being received in the recess 55 in the next lower section locking nut 53. The interlocked shaped charge assemblies are supported by a bottom plate 56 resting on the top of the bull plug 13. The shaped charge assemblies are maintained in rigid axial alignment by a cap locking ring 57 threadably engaged inside the collar 20 of the top closing member 14. The cap locking ring presses down on the upper surface of the spindle 41 which is received in the bore opening 21 of the top closing member.

The present invention achieves a more complete outthrough of the casing at the points of intersection of the vertical and horizontal jets by virtue of a novel and unique arrangement for detonating the first, second and third explosive sections.

The detonating arrangement may be best explained by 6 describing the operation of the complete explosive apparatus. The complete explosive apparatus is positioned in the length of casing desired to be cut as illustrated in FIG. 1. Electrical energy supplied from the surface through the lead wires 30 detonates the blasting cap 29 which detonates the booster blasting cap 32 which, in turn, detonates the fuse 34. The detonation Wave progresses down the fuse to cause detonation of the booster plug 48 in the first explosive section, in turn detonating the booster ring 39 therein and the first explosive section 36. The resulting annular jet penetrates the housing wall 12 and severs the casing 10 in a horizontal plane. No detonation of the second explosive section 49 is initiated upon passage of the detonating wave through that portion of the fuse surrounded by the explosive section 49, the relatively thick solid Wall of the locking pin 54 preventing transmission of a shock wave of sufiicient intensity to initiate the second explosive section. When the detonation wave reaches the booster plug in the third explosive section 36a, the plug 40 detonates and, in turn, detonates the booster ring 39 and the third explosive section, which produces a jet that severs the casing on another horizontal plane opposite the third section. Upon detonation of the third explosive section, the booster ring contacting the top surface thereof is detonated and, in turn, detonates the adjacent booster rings axially aligned and centered in the second explosive section, and therefrom the second explosive section is detonated. The sequence of detonation of the explosive sections allows a ring segment to be cut from the well casing by the first and third explosive sections and this ring segment is in turn out into small rectangular sections by jets from the vertically acting second explosive section. By detonating each of the beforementioned explosive sections at a different time and in the order described, there is no inter- I ference developed between the vertical and horizontal acting jets. A clean, complete cut-through of the casing occurs at the intersection of the lines along which the vertical and horizontal jets act. Provision of alternate explosive confining disks 43 and an intermediate spacing disk 44 between the lower plane surface of each annular explosive section and the top surface of each adjacent frusto-spherical explosive section prevents sympathetic detonation of the frusto-spherical section from occurring.

When a plurality of the second explosive assemblies 48 are axially aligned with a first explosive assembly 35 and coupled into a unitary explosive apparatus as seen in FIG. 1, the individual explosive sections are detonated in the order first, third, second, fifth, fourth, seventh, sixth, ninth, eighth, and so on.

It is to be understood that the explosive shaped charge apparatus herein described and illustrated is subject to wide modification without departing from the scope and spirit of this invention. For example, an open skeleton framework carrier may be used to lower the assembled explosive sections into a dry well bore. Means to interconnect the explosive sections into a unitary assembly other than that described herein may be used. It is contemplated that the stack of booster ring explosive in the second explosive section may be omitted, save only the one booster ring in detonating relation with the third explosive section. Also contemplated is the omission of the booster ring in the first and third explosive sections and use of the booster plug, which may be increased in size, to initiate the explosive charges. Other modifications will be apparent to those skilled in the art. Accordingly, the specific embodiments herein described and depicted are to be considered as merely illustrative and not as restricting the scope of the following claims.

I claim:

1. In an apparatus for cutting sections from well cas ing and the like: a first, a second and a third section of detonating explosive, said sections being axially aligned, said first and said third sections each being generally annular and having a circumferential groove in the periphery thereof, said second section being substantially frustospherical and having a multiplicity of longitudinal grooves therein, said sections being spaced along their common axes and arranged so that, if said sections were separately detonated with quiet intervals between detonations, the cuts in said casing produced by the jets of said second detonating explosive section would intersect the cuts in said casing produced by the jets of said first and third detonating explosive sections; and means including an explosive train arranged for first detonating said first and third sections and thereafter detonating said second section.

2. In an apparatus for cutting sections from well casing and the like: a first, a second and a third section of detonating explosive, said sections being axially aligned, said first and said third section each being generally annular and having a circumferential groove in the periphery thereof, said second section being substantially frustospherical and having a multiplicity of longitudinal grooves therein, said sections being spaced along their common axes and arranged so that, if said sections were separately detonated with quiet intervals between detonations, the cuts in said casing produced by the jets of said second detonating explosive section would intersect the cuts in said casing produced by the jets of said first and third detonating explosive sections; and means including an explosive train arranged for detonating said sections in rapid succession in the order first, third and second.

3. In an apparatus for cutting sections from well casing and the like: a first, a second and a third section of detonating explosive, said sections being axially aligned, said first and said third sections being generally annular with concave recesses in the periphery thereof, said second section being substantially frusto-spherical with a multiplicity of longitudinally convex and circumferentially concave recesses in the spherical surface; means lining said recesses in said first, second and third sections, said sections being spaced along their common axes and arranged so that, if said sections were separately detonated with quiet intervals between detonations, the cuts in said casing produced by the jets of said second detonating explosive section would intersect the cuts in said casing produced by the jets of said first and third detonating explosive sections; and means including an explosive train for detonating said sections in rapid succession arranged to detonate said sections in the order first, third and second.

4. In an apparatus for cutting sections from well casing and the like: a first, a second and a third section of detonating explosive, said sections being axially aligned, said first and said third sections being generally annular with concave recesses in the periphery thereof, said second section being substantially frusto-spherical with a multiplicity of longitudinally convex and circumferentially concave recesses in the spherical surface; means lining said recesses in said first, second and third sections; said sections being spaced along their common axes and arranged so that, if said sections were separately detonated with quiet intervals between detonations, the cuts in said casing produced by the jets of said second detonating explosive section would intersect the cuts in said casing produced by the jets of said first and third detonating explosive sections; and each of said sections providing an axial opening therethrough, an open bore conduit received in each of said openings, and said conduit in said first and third sections providing means to transmit a detonation impulse from said open bore to said first and third sections, and said conduit in said second section providing means to prevent transmission of a detonation impulse from said open bore to said second section; means to initiate detonation of said second section from detonation of said third section; and means including an explosive train to initiate a detonation impulse in said open bore 8 conduit so that detonation of said sections occurs in the order of first, third and second.

5. In an apparatus for cutting sections from well casing and the like: a first, a second and a third section of detonating explosive, said sections being axially aligned, said first and said third sections being generally annular with concave recesses in the periphery thereof, said second section being substantially frusto-spherical with a multiplicity of longitudinally convex and circumferentially concave recesses in the spherical surface; means lining said recesses in said first, second and third sections; said sections being spaced along their common axes and arranged so that, if said sections were separately detonated with quiet intervals between detonations, the cuts in said casing produced by the jets of said second detonating explosive section would intersect the cuts in said casing produced by the jets of said first and third detonating explosive sections; and a plurality of explosive units comprising unitary assemblies of duplicates of said first and second sections axially aligned and associated with said first three sections; and means for detonating said sections including a detonating assembly capable of detonating said sections in the order of first, third, second, fifth, fourth, seventh, sixth, ninth, eighth and so on.

6. In an apparatus for cutting sections from well casing and the like: a first, a second and a third section of detonating explosive, said sections being axially aligned, said first and said third sections being generally annular with concave recesses in the periphery thereof, said second section being substantially frusto-spherical with a multiplicity of longitudinally convex and circumferentially concave recesses in the spherical surface; means lining said recesses in said first, second and third sections; explosion confining means on the outer surfaces of said first section and outer surfaces of said third section and between said second and third sections; inert spacing means inserted between said first section and said second section, and said explosion confining means, spacing means and explosive sections being associated along their common axes so that, if said sections were separately detonated with quiet intervals between detonations, the cuts in said casing produced by the jets of said second section would intersect the cuts in said casing produced by the jets of said first and third sections; and means for detonating said sections in rapid succession including an explosive train arranged to detonate said sections in the order first, third and second.

7. In an apparatus for cutting sections from Well casing and the like: a first, a second and a third section of detonating explosive, said first and said third sections being generally annular with concave recesses in the periphery thereof, said second section being substantially frusto-spherical with a multiplicity of longitudinally conveX and circumferentially concave recesses in the spherical surface; means lining said recesses in said first, second and third sections; an inert explosion confining member on the outer surface of said first section and outer surface of said third section; inert detonation retarding means interposed between said first section and said second section, and said confining member; and said detonation retarding means and explosive sections being associated along their common axes so that, if said sections were separately detonated with quiet intervals between detonations, the cuts in said casing produced by the jets of said second section would intersect the cuts produced by the jets of said first and third sections; a tubular shield means extending axially through said sections, confining members and detonation retarding means, said shield means providing a first lateral opening communicating the bore of said shield means with said first section and a second lateral opening communicating the bore of said shield means with said third section; booster explosive plugs in said first lateral opening and in said second lateral 9 i@ opening; booster explosive rings surrounding said shield References Cited in the file of this patent means in said first and third sections in detonating rela- UNITED STATES PATENTS tionship to said booster explosive plugs; booster explosive rings surrounding said shield means in said second 21682834 Church et July 6, 1954 section and in said detonation confining member in such 5 2,708,408 Sweetman May 171 1955 a relationship to said third section as to initiate detona- 2,782,715 Udry 1957 tion of said second section in response to detonation of 2,831,429 Moore 1958 said third section; a detonating fuse disposed in said 2,856,850 Church 211 1958 tubular shield means; and means to detonate said fuse in i such a manner as to initiate detonation of said explosive 10 2,935,020 Howard et a1 y 1960 sections in the order first, third and second. 

